1. Field of the Invention
This invention relates to a method of an apparatus for detecting whether or not an image forming optical system is focused on an object, and more particularly to a focus detecting method and apparatus of the so-called TTL (Through The Lens) type in which the focusing condition of the optical system is detected by sensing the imaging condition of an image of the object formed by the optical system.
2. Description of the Prior Art
For the above-described or so-called TTL type focus detecting method and apparatus, there have already been put forward a variety of proposals. Of these, the most typical one is that the non-linear output versus illumination response characteristic of a CdS element is utilized in sensing a change in the sharpness of an image of an object formed by an image forming optical system on the prescribed focal plane, to detect the focusing condition of the optical system to that object. Particularly, regarding this apparatus, various improvements have been achieved. However, this method of detecting the condition of sharpest focus of the image forming optical system, by measuring the sharpness of image as a function of the non-linear response characteristics of CdS elements, not only suffers from the difficulty of achieving a much desired increase of the sensitivity of the element, but also has the drawback that the condition when the image is in highest sharpness, or when the output of the element reaches a peak value, is difficult to detect with high precision. Thus, it is unavoidable that these factors lead to place a relative low limit on its detecting performance.
As a method of detecting the focusing condition of the optical system, on the other hand, besides the positive and direct measurement of the sharpness of the image, it is also possible to otherwise detect the focusing condition. As is disclosed, for example, in Japanese Laid-Open Patent Specifications Sho 51-15432 and Sho 52-138924, on the exit pupil plane of the image forming optical system there are prescribed two areas lying in almost symmetry to each other with respect to the optical axis of the optical system. Two light beams from these two prescribed exit pupil areas form two images on the prescribed focal plane at respective positions. By sensing the change in the distance between the positions, or, in extreme cases, the coincidence or non-coincidence of the two images, whether or not the image forming optical system is in focus for the object can be detected. And, since, in this case, the condition of sharp focus is determined as a function of not the image sharpness, but the difference in position between the two images, a particular advantage that focus detection can be performed with relative ease, while still permitting accuracy to be greatly increased, is fully expected therefrom. Another advantage of this method is that not only can the condition of sharpest focus be detected, but also, when the lens system is not in focus, the detection of a direction in which the optical system is in being moved, that is, either near focus or far focus can be indicated along with a determination of the amount of movement by which the optical system is brought into sharp focus. Therefore, the use of this idea in the TTL type focus detecting method gives rise to a high possibility of achieving remarkable improvements, with respect to various aspects, over the former idea of direct measurement of the image sharpness itself.
However, even when this latter idea is put into practice in the field of art of cameras and other optical instruments, while nevertheless permitting focus detection to be performed with fully satisfactory accuracy and with high reliability, there are necessary requirements for much more improvement.
According to the proposal, for example, in the above-cited Japanese Laid-Open Patent Specification Sho 51-15432, the above-described two images are sensed by respective groups of large numbers of photosensitive elements. By comparing the electrical outputs from the two photosensitive element groups with each other, the detection of a mutual positional relationship of the two images can be effected. And, for this purpose, it is suggested that the comparator circuit may be constructed in the form of, for example, a diode bridge circuit so that as all the photosensitive elements are paired up in positional correspondence between the two groups, the total sum of coincidence and incoincidences (or balances and unbalances) of the outputs of the photosensitive elements in every pair can be determined. However, it has been proven that such a simple comparison method and structure of the comparator circuit do not suffice to achieve what may be called an accurate focus detection, without encountering very difficult problems.
Further, according to another proposal as in Japanese Laid-Open Patent Specification Sho 52-138924, the above-described two images are received by respective photo-cells of the type in which their resistance values vary with the change in the positions of the images on the image-receiving surfaces there of (or, image position sensors). Then, the balance and unbalance of the resistance values of these two cells is detected through a bridge circuit to determine the mutual position as relationship of the two images. Even with such an improved comparison method and circuit structure, a much desired increase in the accuracy of focus detection is still very difficult to achieve.